Flatten each cardboard roll lengthwise and crease the sides. Open each tube back up.
Flatten each tube lengthwise on the opposite side. Crease the sides so they line up with the previous creases. Each roll will now have four sides and four corners.
Cut the creased rolls with scissors to make shorter tubes. Cut toilet paper rolls in half; cut paper towel rolls into four pieces.
Cut along each crease on one end of tube section. Slice each crease about 5/8 of an inch to make tiny flaps.
Fold the flaps down toward the center, creasing them well. Interlock the flaps like a box top.
Fill your seed-starter pots with potting soil. Plant your seeds.